1. Field of the Invention
This invention relates to an optical recording medium in which an information is optically recorded, and more particularly to a multi-layer optical recording medium having a number of recording layers.
2. Description of the Prior Art
Generally, an optical recording medium has been fabricated in a disc type having a fast random access ability. The disc-type optical recording medium is classified into a read-only type and a recordable type. Also, the recordable disc is subdivided into a write-once-read-only(WROM) type which is recordable once and a rewritable type which is recordable repeatedly. The read-only type disc includes a compact disc-read only memory(CD-ROM) and a digital versatile disc-read only memory(DVD-ROM), etc. Furthermore, the WORM type disc includes a recordable compact disc(CD-R) and a recordable digital versatile disc(DVD-R), etc. and the rewritable disc includes a rewritable compact disc(CD-RW), a rewritable digital versatile disc(DVD-RW or DVD-RAM) and a magneto-optical disc MOD, etc.
Such an optical recording medium requires a large recording capacity enough to record a large information such as a moving picture. In order to satisfy the requirement for a large recording capacity, there has been developed a double-face optical recording medium wherein a track pitch is narrow and a recording face exists in each of the upper and lower surfaces thereof. For example, U.S. Pat. No. 4,450,553 discloses a multi-layer optical recording medium having at least two recording layers alternately disposed with light transmission layers used as a substrate. The multi-layer optical recording medium is noticed in that it can assure a larger recording capacity than a double-face optical recording medium. According to the U.S. Patent, the recording layer used for the multi-layer optical recording medium has a light reflection characteristic as well as a light transmission characteristic. This is caused by a fact that, when a certain recording layer is accessed, the upper recording layers thereof must transmit a light while the certain recording layer must reflect a received light. Accordingly, the recording layers of the multi-layer optical recording medium is formed of a semi-permeable material having the two optical characteristics. The semi-permeable material has an intrinsic light reflection coefficient Rc expressed as a certain constant value to reflect a constant ratio of incident light amount irrespective of an intensity of the incident light as shown in FIG. 1. The intrinsic light reflection coefficient Rc of the semi-permeable material becomes different depending upon a type of semi-permeable material. Due to the recording layers of the semi-permeable material, a light loss is generated at the multi-layer optical recording medium. For instance, when a second recording layer 8 included in a two-layer optical disc as shown in FIG. 2 is accessed, that is, when a light spot is formed on the second recording layer 8, a light incident to the second recording layer 8, via a first light transmission layer 2, a first recording layer 4 and a second light transmission layer 6, from a light source(not shown) is lost by an amount corresponding to a reflection coefficient of the first recording layer 4. Also, a reflective light reflected by the second recording layer 8 is more reduced by an amount corresponding to a light transmissivity of the second recording layer 8. Furthermore, a reflective light passing through the second light transmission layer 6, the first recording layer 4 and the first light transmission layer 2 from the second recording layer 8 is again reduced by an amount corresponding to a light reflection coefficient of the first recording layer 1. Herein, it is assumed that an absorption coefficient of each of the first and second recording layers 4 and 8 is xe2x80x9c0xe2x80x9d, a light reflection coefficient R1 of the first recording layer 4 is 0.3, that is, a light transmissivity (1xe2x88x92R1) of the first recording layer 4 is 0.7, and a light reflection coefficient R2 of the second recording layer 8 is 0.7. In this case, a light amount reflected by the second recording layer 8 and emitting from the first light transmission layer 2 becomes equal to xe2x80x9c0.343=(1xe2x88x92R1)xc3x97R2xc3x97(1xe2x88x92R1)=0.7xc3x970.7xc3x970.7xe2x80x9d times of the light amount incident to the first light transmission layer 2. On the other hand, when the first recording layer 4 of the two-layer optical disc in FIG. 2 is accessed, a reflective light amount emitting from the first light transmission layer 2 becomes equal to 0.3 times of the light amount incident to the first light transmission layer 2. In other words, when the first recording layer 4 is accessed, a light loss amount becomes 70% of the incident light amount.
In order to increase a reflective light amount emitted from the first light transmission layer 2 when the first recording layer 4 is accessed, the reflection coefficient R1 of the first recording layer 4 can be heightened. For instance, if the reflection coefficient of the first recording layer 4 has been set to 40%, then a reflective light amount emitted from the first light transmission layer 2 when the first recording layer 4 is accessed, increases into 40% of the light amount incident to the first light transmission layer 2. But, a reflective light amount emitted from the first light transmission layer 2 when the second recording layer 8 is accessed, decreases dramatically due to a low light transmissivity (1xe2x88x92R1) of the first recording layer 4. In other words, a reflective light amount emitted from the first light transmission layer 2 when the second recording layer 8 is accessed, becomes equal to xe2x80x9c0.252=(1xe2x88x92R1)xc3x970.7xc3x97(1xe2x88x92R1)=0.6xc3x970.7xc3x970.6xe2x80x9d times of a light amount incident to the first light transmission layer 2. As described above, a reflective light amount when the second recording layer 8 is accessed, is dramatically reduced in comparison to a reflective light amount when the first recording layer 4 is accessed. Accordingly, a signal detected from the second recording layer 8 also has a dramatically lower voltage level than a signal detected from the first recording layer 4. In other words, the signal detected from the first recording layer 4 can have a reproducible voltage level, but the signal detected from the second recording layer 8 fails to obtain the reproducible voltage level. As a result, a data recorded on the second recording layer 8 can not be read out by an optical pickup. In view of this, it is necessary to provide a multi-layer optical recording medium with a high light efficiency so that all the multiple recording layers can be accessed.
Accordingly, it is an object of the present invention to provide an optical recording medium with a number of recording layers that is adaptive for enhancing a light efficiency, and a method of fabricating the same.
Further object of the present invention is to provide an optical recording medium with a number of recording layers that is adaptive for improving a responsibility in the information reproduction, and a method of fabricating the same.
Still further object of the present invention is to provide an optical recording medium that is adaptive for increasing a recording capacity, and a method of fabricating the same.
In order to achieve these and other objects of the invention, in an optical recording medium with a number of recording layers according to one aspect of the present invention, at least one of the number of recording layers is made from a non-linear material having a non-linear optical characteristic.
A method of fabricating an optical recording medium with a number of recording layers according to another aspect of the present invention includes the steps of providing a number of substrates; forming a recording layer on each of the number of substrates in such a manner that a material with a non-linear optical characteristic having a different reflective coefficient depending on a light intensity is formed on at least one of the number of substrates; and bonding the same in such a manner that the number of substrates are disposed.